Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-MRAM). STT-MRAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction.
A conventional magnetic tunneling junction (MTJ) may be used in a conventional STT-MRAM. The MTJ includes a pinned layer, a free layer and a tunneling barrier layer between the pinned and free layers. The MTJ typically resides on a substrate and may include seed and capping layer(s) as well as an antiferromagnetic (AFM) pinning layer adjoining the pinned layer. A bottom contact below the MTJ and a top contact on the MTJ may be used to drive current through the MTJ in a current-perpendicular-to-plane (CPP) direction.
The pinned layer and the free layer are magnetic. The magnetization of the pinned layer is fixed, or pinned, in a particular direction. The free layer has a changeable magnetization. The free layer and the pinned layer may each be a single layer or include multiple layers. The pinned layer and free layer may have their magnetizations oriented perpendicular to the plane of the layers (perpendicular-to-plane) or in the plane of the layers (in-plane).
To switch the magnetization of the conventional free layer, a current is driven perpendicular to plane. The current becomes spin polarized and exerts a spin torque on the magnetic moment of the free layer. When a sufficient current is driven from the top contact to the bottom contact, the magnetization of the conventional free layer may switch to be parallel to the magnetization of a conventional bottom pinned layer. When a sufficient current is driven from the bottom contact to the top contact, the magnetization of the free layer may switch to be antiparallel to that of the bottom pinned layer. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ.
To fabricate conventional MTJs in a STT-MRAM, the layers in the MTJ are blanket deposited across the surface of the substrate. These layers form an MTJ stack. Layers for the pinned layer, the nonmagnetic spacer layer and the free layer are all included in the MTJ stack. Additional layers such as seed and/or capping layers may also be part of the MTJ stack. Once the entire MTJ stack is deposited, a mask is provided. The mask covers the regions where the MTJs are to be formed and has apertures between the MTJs. The exposed portions of the MTJ stack are then removed. This removal may be accomplished via processes such as reactive ion etches (RIEs) and/or ion milling. Thus, the individual MTJs are defined from the MTJ stack. Fabrication of the STT-MRAM may then be completed. For example, insulating refill, conductive lines, and other components maybe formed.
High density STT-MRAM devices are increasingly desired. The spacing between memory cells and, therefore, the conventional MTJs continues to shrink. The height of the MTJ stack does not necessarily decrease with the reduction in spacing. Consequently, the aspect ratio (height divided by width or height divided by length) may increase. As the spacing between MTJs decreases and the aspect ratio increases, fabrication may become more challenging. For example, ion milling may be incapable of defining the MTJs at smaller spacing and higher aspect ratios. Further, because the MTJs have multiple layers of various materials, no single RIE chemistry is currently available for fabrication. Accordingly, what is needed is a method and system that may improve fabrication of spin transfer torque based memories. The method and system described herein address such a need.